How to use the beets.library function in beets

def modify_items(lib, mods, dels, query, write, move, album, confirm):
    """Modifies matching items according to user-specified assignments and
    deletions. `mods` is a list of "field=value" strings indicating
    assignments. `dels` is a list of fields to be deleted.
    """
    # Parse key=value specifications into a dictionary.
    model_cls = library.Album if album else library.Item
    fsets = {}
    for mod in mods:
        key, value = mod.split('=', 1)
        fsets[key] = model_cls._parse(key, value)

    # Get the items to modify.
    items, albums = _do_query(lib, query, album, False)
    objs = albums if album else items

    # Apply changes *temporarily*, preview them, and collect modified
    # objects.
    print_('Modifying %i %ss.' % (len(objs), 'album' if album else 'item'))
    changed = set()
    for obj in objs:
        for field, value in fsets.iteritems():
            obj[field] = value

def aunique_benchmark(lib, prof):
    def _build_tree():
        vfs.libtree(lib)

    # Measure path generation performance with %aunique{} included.
    lib.path_formats = [
        (library.PF_KEY_DEFAULT,
         Template('$albumartist/$album%aunique{}/$track $title')),
    ]
    if prof:
        cProfile.runctx('_build_tree()', {}, {'_build_tree': _build_tree},
                        'paths.withaunique.prof')
    else:
        interval = timeit.timeit(_build_tree, number=1)
        print('With %aunique:', interval)

    # And with %aunique replaceed with a "cheap" no-op function.
    lib.path_formats = [
        (library.PF_KEY_DEFAULT,
         Template('$albumartist/$album%lower{}/$track $title')),
    ]
    if prof:
        cProfile.runctx('_build_tree()', {}, {'_build_tree': _build_tree},

def aunique_benchmark(lib, prof):
    def _build_tree():
        vfs.libtree(lib)

    # Measure path generation performance with %aunique{} included.
    lib.path_formats = [
        (library.PF_KEY_DEFAULT,
         Template('$albumartist/$album%aunique{}/$track $title')),
    ]
    if prof:
        cProfile.runctx('_build_tree()', {}, {'_build_tree': _build_tree},
                        'paths.withaunique.prof')
    else:
        interval = timeit.timeit(_build_tree, number=1)
        print('With %aunique:', interval)

    # And with %aunique replaceed with a "cheap" no-op function.
    lib.path_formats = [
        (library.PF_KEY_DEFAULT,
         Template('$albumartist/$album%lower{}/$track $title')),
    ]
    if prof:
        cProfile.runctx('_build_tree()', {}, {'_build_tree': _build_tree},

autotag.apply_metadata(albuminfo, item_to_trackinfo)
                changed = False
                # Find any changed item to apply Beatport changes to album.
                any_changed_item = items[0]
                for item in items:
                    item_changed = ui.show_model_changes(item)
                    changed |= item_changed
                    if item_changed:
                        any_changed_item = item
                        apply_item_changes(lib, item, move, pretend, write)

                if pretend or not changed:
                    continue

                # Update album structure to reflect an item in it.
                for key in library.Album.item_keys:
                    album[key] = any_changed_item[key]
                album.store()

                # Move album art (and any inconsistent items).
                if move and lib.directory in util.ancestry(items[0].path):
                    self._log.debug(u'moving album {}', album)
                    album.move()

def read_item(self, path):
        """Return an `Item` read from the path.

        If an item cannot be read, return `None` instead and log an
        error.
        """
        try:
            return library.Item.from_path(path)
        except library.ReadError as exc:
            if isinstance(exc.reason, mediafile.FileTypeError):
                # Silently ignore non-music files.
                pass
            elif isinstance(exc.reason, mediafile.UnreadableFileError):
                log.warning(u'unreadable file: {0}', displayable_path(path))
            else:
                log.error(u'error reading {0}: {1}',
                          displayable_path(path), exc)

# Start generating the script
    yield "_beet() {\n"

    # Command names
    yield "  local commands='%s'\n" % ' '.join(command_names)
    yield "\n"

    # Command aliases
    yield "  local aliases='%s'\n" % ' '.join(aliases.keys())
    for alias, cmd in aliases.items():
        yield "  local alias__%s=%s\n" % (alias, cmd)
    yield '\n'

    # Fields
    yield "  fields='%s'\n" % ' '.join(
            set(library.ITEM_KEYS + library.ALBUM_KEYS))

    # Command options
    for cmd, opts in options.items():
        for option_type, option_list in opts.items():
            if option_list:
                option_list = ' '.join(option_list)
                yield "  local %s__%s='%s'\n" % (option_type, cmd, option_list)

    yield '  _beet_dispatch\n'
    yield '}\n'

def add_media_field(self, name, descriptor):
        """Add a field that is synchronized between media files and items.

        When a media field is added ``item.write()`` will set the name
        property of the item's MediaFile to ``item[name]`` and save the
        changes. Similarly ``item.read()`` will set ``item[name]`` to
        the value of the name property of the media file.

        ``descriptor`` must be an instance of ``mediafile.MediaField``.
        """
        # Defer impor to prevent circular dependency
        from beets import library
        mediafile.MediaFile.add_field(name, descriptor)
        library.Item._media_fields.add(name)

def _rep(obj, expand=False):
    """Get a flat -- i.e., JSON-ish -- representation of a beets Item or
    Album object. For Albums, `expand` dictates whether tracks are
    included.
    """
    out = dict(obj)

    if isinstance(obj, beets.library.Item):
        del out['path']

        # Get the size (in bytes) of the backing file. This is useful
        # for the Tomahawk resolver API.
        try:
            out['size'] = os.path.getsize(util.syspath(obj.path))
        except OSError:
            out['size'] = 0

        return out

    elif isinstance(obj, beets.library.Album):
        del out['artpath']
        if expand:
            out['items'] = [_rep(item) for item in obj.items()]
        return out